Surveillance of near-earth space with a radar network

Transmit fields-of-view, orbits and detection points for three exemplary objects.

With the support of the aerospace management of the German Aerospace Center (DLR-RFM), Fraunhofer FHR is working on concepts for the surveillance of near-Earth orbit using a network of radar systems. Scientists analyze the efficiency of various concepts and devise potential solutions for the anticipated technical challenges.

Modern society relies, to a growing extent, on the utilization of near-Earth space, e.g. for satellite-based communication services and Earth observation. The volume of space debris continues to grow unabated, partly due to this intensive utilization. Contributors to space debris include the disused upper stages of carrier rockets, satellites that no longer function and residue from rocket fuels and small objects that have become separated from satellites. Two particular events, namely the collision between the two satellites Iridium 33 and Cosmos-2251 in 2009 and the intentional destruction of a satellite in 2007 each led to a large increase in the debris population.

Space debris jeopardizes the utilization of near-Earth space. A collision between even a relatively small debris particle and a satellite can lead to the destruction of the satellite, due to their high speeds. This problem, among others, has in recent years made clear the importance of gaining knowledge pertaining to the population of satellites and space debris, as it can be used to plan evasive maneuvers for satellites to reduce the risk of a collision.

It is in this context, that the aerospace management of the German Aerospace Center (DLR-RFM) commissioned Fraunhofer FHR with the development of the GESTRA system. With GESTRA, Germany will, for the first time, have the ability to carry out a large-scale, radar-based search for debris particles and satellites in near-Earth space (near-Earth orbital paths), and it will also be possible to determine the orbits of detected objects.

At Fraunhofer FHR, the scientists are also looking beyond the GESTRA system to a time in the future when a network of radar systems will jointly search near-Earth space for debris particles and satellites and determine the orbits of these objects. This takes place within the framework of the DLR-RFM-funded project »A Network of Radars with Array Antenna for Space Surveillance« which extends into 2019.

Here, the scientists investigate a number of overarching questions, such as »Should the radar systems in the network be positioned near to each other, i.e. within a distance of a couple of hundred meters?«, this type of network is known as a ‘local radar network’ or »Would it be better to distribute the radar systems over several hundred or even thousand kilometers?«, researchers refer to this as a ‘radar network of medium extent’. Both approaches have their own advantages: in a local network, the data of the receivers can, due to their physical proximity, be combined in a very elegant manner so that even objects with a small radar cross section can be detected. A network of medium extent, on the other hand, facilitates the acquisition of data from different angles. It is expected that the combination of this data will allow for more precise measurements of position and speed.

The researchers carry out performance tests to compare the respective strengths of the different network types. Here, they model the typical radar-related characteristics of a radar network, such as detection probabilities and estimation accuracies. The most important factor, however, is the orbit determination ability. For this reason, part of the project is dedicated to assessing the orbit determination accuracy of various radar network concepts with to the goal of identifying the most promising approach.

Figure 1 shows a result of these performance tests. The figure shows the extent to which an object can – based on certain assumptions – be better detected at a height of 700 km with an exemplary radar network when bistatic signal paths are also evaluated together with the three monostatic signal paths. Black dots indicate the locations of the individual radar systems in the network. Figure 2 shows the coverage area of the same network and simulates target detections which serve as input variables for orbit determination.

In addition to the performance tests, the scientists also carry out initial investigations to identify technical challenges that may occur. In particular, they closely examine the synchronization of the subsystems, the design of the data processing in the local network and the evaluation of bistatic signal paths in a medium-sized network.